A preferred application for the present invention is in high voltage alternating current (AC) three phase circuit breakers and reclosers, the latter being a type of circuit breaker. Therefore, the background of the invention is described below in connection with such devices. However, it should be noted that, except where they are expressly so limited, the claims at the end of this specification are not intended to be limited to applications of the invention in a high voltage three phase AC circuit breaker.
A high voltage circuit breaker is a device used in the distribution of three phase electrical energy. When a sensor or protective relay detects a fault or other system disturbance on the protected circuit, the circuit breaker operates to physically separate current-carrying contacts in each of the three phases by opening the circuit to prevent the continued flow of current. A recloser differs from a circuit breaker in that a circuit breaker opens a circuit and maintains the circuit in the open position indefinitely, whereas a recloser may automatically open and reclose the circuit several times in quick succession to allow a temporary fault to clear and thus, avoid taking the circuit out of service unnecessarily.
The major components of a circuit breaker or recloser include the interrupters, which function to open and close one or more sets of current carrying contacts housed therein; the operating or driving mechanism, which provides the energy necessary to open or close the contacts; the arcing control mechanism and interrupting media, which create an open condition in the protected circuit; one or more tanks for housing the interrupters; and the bushings, which carry the high voltage electrical energy from the protected circuit into and out of the tank(s). In addition, a mechanical linkage connects the interrupters and the operating mechanism.
Circuit breakers may differ in the overall configuration of these components. However, the operation of most circuit breakers is substantially the same regardless of their configurations. For example, a circuit breaker may include a single tank assembly which houses all of the interrupters. U.S. Pat. No. 4,442,329, Apr. 10, 1984, "Dead Tank Housing for High Voltage Circuit Breaker Employing Puffer Interrupters," discloses an example of the single tank configuration. Alternatively, a separate tank for each interrupter may be provided in a multiple tank configuration. An example of a multiple tank configuration is depicted in FIG. 1.
As shown in FIG. 1, a prior art circuit breaker assembly 1 includes three cylindrical metal tanks 3. The three cylindrical tanks 3 form a common tank assembly 4 which is preferably filled with an inert, electrically insulating gas such as SF.sub.6. The tank assembly 4 shown in FIG. 1 is referred to as a "dead tank" in that it is at ground potential. Each tank 3 houses an interrupter (not shown in FIG. 1). The operation of an interrupter is described below. The interrupters are provided with terminals which are connected to respective spaced bushing insulators. The bushing insulators are shown as bushing insulators 5a and 6a for the first phase; 5b and 6b for the second phase; and 5c and 6c for the third phase. Associated with each pole or phase is a current transformer 7.
SF.sub.6 breaker bushings are an integral part of the breaker, both electrically and mechanically. They are not designed or used as general purpose apparatus bushings. SF.sub.6 breaker bushings are designed to support and insulate high voltage line connections and carry power into the grounded tank of the circuit breaker.
In high voltage circuit breakers, the pairs of bushings for each phase are often mounted so that their ends have a greater spacing than their bases to avoid breakdown between the exposed conductive ends of the bushings. One means for achieving the desired spacing has been to use conical bushings such that the terminal ends of the bushings have smaller diameters than their respective bases. For example, FIG. 1A shows a high voltage circuit breaker with conical bushings 90a-c and 92a-c. The conical bushings are angled away from each other to provide an adequate air gap (AG) between their ends so that in the event of a flashover or significant current leakage, the resulting breakdown is grounded in the dead tank. Therefore, it is desirable that the spacing between the terminal portions of the bushings, i.e., air gap, be greater than the length of the bushing. As circuit breakers become more compact, the size and spacing of the bushings become a critical design feature of the circuit breaker.
A longitudinal cross section of a typical conical bushing is shown in FIG. 1B. A high voltage conductor 100 is surrounded by an insulator 101 with weather sheds 102. The conductor 100 is electrically coupled between an interrupter and the protected circuit. The insulator 101 of the SF.sub.6 breaker bushing is shaped and sized to accommodate an internal grading shield 103 which optimizes dielectric strength (internally and externally). The shield 103 is uniquely shaped to grade the voltage field in the air along the exterior length of the bushing as well as inside where the bushing conductor 100 enters the grounded tank. The bushing conductor 100, running through the hollow, SF.sub.6 -filled insulator 101, creates a radial stress through the bushing. This stress is higher at the entrance to the grounded tank. Therefore, the shield 103 reduces the stress on the insulator to improve the reliability of the bushing. The weather sheds 102 on the external surface of the bushing resist the effects of rain and surface dirt to maintain good dielectric conditions.
Traditionally, bushing insulators have been made from porcelain or a cast epoxy. Typically, the weather sheds are designed so that water rolls off the sheds keeping the underside of the sheds substantially dry. However a significant portion of the insulator surface can become wet or degraded by environmental pollution. The resulting weakening of the dielectric can cause leakage and flashover conditions.
An additional drawback of porcelain or cast epoxy bushings is that they are relatively brittle and, therefore, subject to damage from external condition that can cause them to shatter so that the SF.sub.6 contained therein explodes. To provide an optimal insulator and a safe and reliable housing for the bushing conductor, the porcelain and cast epoxy insulators are produced with a relatively thick wall (i.e., about 1 inch). The increased thickness further narrows the air gap, increases the weight of the bushings, and increases the cost of the bushings.
Therefore, a composite bushing has been developed that provides the following advantages over traditional bushings: non-brittle behavior, reduced weight and wall thickness, pollution resistance, and improved wet electrical capability. A longitudinal cross section of a composite bushing is shown in FIG. 1C. Composite bushings insulators are made up of a fiberglass reinforced tube 110 protected by a silicone rubber housing 112. These bushings have a straight cylindrical composite tube with aluminum end flanges 114 and 116 and room temperature, vulcanized (RTV) silicone rubber weather sheds 120. The RTV silicone rubber has a hydrophobic surface due to oil films that naturally form on the rubber surface.
The composite bushings are produced by using an injection molding technique in which the a single mold forms a single section of the housing 112 at a time. This process is both time consuming and relatively inefficient in that each section of the housing must also be molded together to form the completed bushing housing. Since the silicone rubber housing is formed from a injection molding process, a specially designed mold would be required to produce the desired conical shape. For many high voltage breakers that require very large bushings, such molds are impractical.
A process for molding rubber using a traveling mold has been commercially exploited. Essentially, the traveling mold is capable of forming plastic or rubber on substantially any shape in a continuous process. Therefore, to improve the performance and reduce the size and weight of high voltage circuit breakers there is a need to design a conical composite bushing that has a housing which can be formed using such a traveling mold.
Two other important elements of a high voltage circuit breaker are the operating mechanism and a mechanical linkage. The operating mechanism that provides the necessary operating forces for opening and closing the interrupter contacts is contained within an operating mechanism housing 9 shown in FIGS. 1 and 1A. The operating mechanism is mechanically coupled to each of the interrupters via a linkage 8.
A cross section of an interrupter 10 is shown in FIGS. 2A-C. The interrupter provides two sets of contacts, the arcing contacts 12 and 14 and the main contacts 15 and 19. Arcing contacts 12 and main contacts 19 are movable, as described in more detail below, to either close the circuit with respective contacts 14 and 15 or to open the circuit. FIG. 2A shows a cross sectional view of the interrupter with its contacts closed, whereas FIG. 2C shows a cross section of the interrupter with the contacts open.
The arcing contacts 12 and 19 of high voltage circuit breaker interrupters are subject to arcing or corona discharge when they are opened or closed, respectively. As shown in FIG. 2B, an arc 16 is formed between arcing contacts 12 and 14 as they are moved apart. Such arcing can cause the contacts to erode and perhaps to disintegrate over time. Therefore, a known practice (used in a "puffer" interrupter) is to fill a cavity of the interrupter with an inert, electrically insulating gas that quenches the arc 16. As shown in FIG. 2B, the gas is compressed by piston 17 and a jet or nozzle 18 is positioned so that, at the proper moment, a blast of the compressed gas is directed toward the location of the arc in order to extinguish it. Once an arc has formed, it is extremely difficult to extinguish it until the arc current is substantially reduced. Once the arc is extinguished as shown in FIG. 2C, the protected circuit is opened thereby preventing current flow.
Typically a bank of shunt capacitors is coupled between the arcing contacts to control the arcing by equalizing the voltages at the respective breaks in a multi-interrupting point type circuit breaker, i.e., one with more than one set of contacts. A capacitor coupled between contacts may also be used in a single-break circuit breaker. The bank of shunt capacitors is typically arranged within a dead tank to surround an arc-extinguishing chamber therein. It is further known to control arcing utilizing pre-insertion or closing resistors, as disclosed in U.S. Pat. No. 5,245,145, Sep. 14, 1993, "Modular Closing Resistor" (assigned to ABB Power T&D Company Inc.).
Voltage and current transients generated during the energization of shunt capacitor banks have become an increasing concern for the electric utility industry in terms of power quality for voltage-sensitive loads and excessive stresses on power system equipment. For example, modern digital equipment requires a stable source of power. Moreover, computers, microwave ovens and other electronic appliances are prone to failures resulting from such transients. Even minor transients can cause the power waveform to skew, rendering these electrical devices inoperative. Therefore, utilities have set objectives to reduce the occurrence of transients and to provide a stable power waveform.
Conventional solutions for reducing the transients resulting from shunt capacitor energization include circuit breaker pre-insertion devices, for example, resistors or inductors, and fixed devices such as current limiting reactors. While these solutions provide varying degrees of mitigation for capacitor bank energization transients, they result in added equipment, added cost, and can result in added reliability concerns.
The maximum shunt capacitor bank energization transients are associated with closing the circuit breaker at the peak of the system voltage waveform, i.e., where the greatest difference exists between the bus voltage, which will be at its maximum, and the capacitor bank voltage, which will be at a zero level. Where the closings are not synchronized with respect to the system voltage, the probability for obtaining the maximum energization transients is high. One solution to this problem is to add timing accuracy to synchronously close the circuit breaker at the instant the system voltage is substantially zero. In this way, the voltages on both sides of the circuit breaker at the instant of closure would be nearly equal, allowing for an effectively "transient-free" energization.
While the concept of synchronous or zero-crossing closing is a simple one, a cost-effective solution has been difficult to achieve, primarily due to the high cost of providing the required timing accuracy in a mechanical system. U.S. Pat. No. 4,306,263, Dec. 15, 1981, entitled "Synchronous Closing System and Latch Therefor," discloses a synchronous closing system wherein the circuit breaker main contacts close within about 1 millisecond of a zero crossing by inhibiting the hydraulic pressure utilized to close the interrupter contacts using a latch controlled mechanism. However, this synchronous closing system is incapable of providing synchronization for each phase or pole individually. Thus, while one phase may be closed synchronously, avoiding transients in that phase of the circuit, harmful transients may be produced by closing the contacts in one or both of the other phases.
One solution might be to utilize three separate operating mechanisms and corresponding linkages to synchronously control the operation of each pole individually. U.S. Pat. No. 4,417,111, Nov. 22, 1983, entitled "Three-Phase Combined Type Circuit Breaker," discloses a circuit breaker having a separate operating mechanism and associated linkage for each of the three phases or poles. However the use of three separate operating mechanisms and associated linkages is expensive and increases the overall size and complexity of the circuit breaker.
U.S. Pat. No. 4,814,560, Mar. 21, 1989, "High Voltage Circuit Breaker" (assigned to Asea Brown Boveri AB, Vasteras, Sweden) discloses a device for synchronously closing and opening a three-phase high voltage circuit breaker so that a time shift between the instants of contact in the different phases can be brought about mechanically by a suitable choice of arms and links in the mechanical linkage. This linkage uses an a priori knowledge of the time required to close and open the interrupter contacts in each of the three phases. The time differences can be accounted for by an appropriate design of the mechanical linkage. However, such a linkage cannot support dynamic monitoring of the zero-crossings for each phase to achieve independent synchronization. Moreover, the mechanical linkage disclosed would require mechanical adjustments over time to account for variations in the circuit breaker performance and operating conditions which often change over time.
A dependent pole operating mechanism has been used in circuit breakers to generate the initial driving forces required to open and close the interrupter contacts. Dependent pole operation refers to the limited capability of the operating mechanism to close or open all three phases of the circuit simultaneously. A prior art example of a dependent pole mechanism and mechanical linkage implemented in a three-phase circuit breaker is shown in FIG. 3. As shown in FIG. 3, operating mechanism 20 provides a single connecting rod 22. Connecting rod 22 is interfaced with linking element 26 via lever 24. Linking elements 25 and 26 preferably form a single linking shaft linking together the terminal portions of each of the three interrupters (not shown in FIG. 3). In operation, the connecting rod 22 is driven up or down thereby pivoting lever 24. As lever 24 pivots, the linking elements 25 and 26 rotate. The linking elements are preferably coupled to bell cranks provided in the terminal portion of the interrupters (not shown in FIG. 3) which pivot in response to the rotation of the linking elements to open and close the contacts of the interrupters. It should be understood that each of the interrupters housed in tanks 3 will open and close simultaneously in response to the movement of connecting rod 22.
Recently an independent pole operating mechanism has been developed which provides an individually controlled driving force for opening and closing each phase of the circuit breaker independently. By utilizing the independent pole operating mechanism, each phase can be dynamically and synchronously switched individually. Thus there is a need to provide a mechanical linkage to operate effectively with the independent pole operating mechanism. To eliminate the necessity of redesigning the entire circuit breaker to implement the new independent pole operating mechanism, it is desirable to cost-effectively adapt existing circuit breaker linkages, such as linkage 8 shown in FIG. 1. Moreover, the mechanical linkage for use with the independent pole operating mechanism should not increase the size of the circuit breaker, or require complex assembly or maintenance steps to ensure that the circuit breaker functions properly.